Published 4:00 am, Sunday, March 12, 2006

Like a trouble-making houseguest who overstays his welcome, the huge mass of Arctic air that's delivered explosive bursts of icy precipitation usually seen around here only in Slurpee cups will hang around for the rest of the weekend.

Despite a sunny respite Saturday, cold, unsettled weather was forecast to deliver more thundershowers -- and the possibility of more snow, sleet, hail or various combinations thereof down to 500 feet or possibly even sea level in some areas today.

"Dangerous driving conditions will continue," said California Highway Patrol Sgt. Wayne Ziese, who spent all day at the scene of a 28-vehicle pileup on Highway 101 in Sausalito that killed two people and injured more than a dozen. "Drivers need to slow down, drive according to the conditions and maintain safe cushions. You've got to be able to slow your car down and stop within the distance you can see."

Showers are expected to disappear tonight and stay away most of Monday, and the temperatures will rise gradually. Rain is expected to return Tuesday, but it will be a significantly warmer storm, with the snow level rising to a more seasonal 4,000 feet, the National Weather Service reported.

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The unusually brisk weather is the product of a massive trough of cold air that's blown in from the Arctic. While such cold air masses are not uncommon in the Bay Area during the winter, this one is unusually large -- covering most of the West Coast and reaching as far southeast as Arizona -- and unseasonably late.

Indeed, the last time it snowed in the Bay Area in March was in 1896, said Brooke Bingaman, a National Weather Service meteorologist in Monterey.

"We don't often see something this big or this late in the season," she said. "It's very unusual."

The combination of extremely cold air, an unstable atmosphere and high moisture create perfect conditions for Mother Nature to generate the sudden, often intense bursts of rain or frozen precipitation the region has seen in recent days.

"Any time it's in the 30 (degree) range, you can look for snow, hail or whatever," Bingaman said.

Even when the forecast places the snow level at 500 feet, intense downpours can deliver a strong burst of extremely cold air that causes snow, sleet or hail at lower elevations. Because all forms of frozen precipitation are anomalies in the Bay Area, she said, people often mistake hail for snow, particularly when it covers the ground. While snow falls in flakes and looks more like small ice shavings, hail is balls of ice, usually small.

If the trees are white, Bingaman said, it's probably snow.

Ziese said it was definitely snow covering the Waldo Grade during Saturday's accident, and that drivers were probably caught by surprise as they came out of the tunnel and down the hill to find themselves on a highway slick with snow.

It's not uncommon to have icy conditions along Highway 101 in that area because of precipitation drainage, the shade and the topography, Ziese said, and Caltrans often spreads sand on the road to prevent accidents.

"We've had dustings of snow there before," he said, "but nothing like this."

Drive carefully

Tips for icy, snowy conditions:

-- Slow down immediately and maintain a speed appropriate for conditions. Speed limits are set for clear, dry conditions and should be reduced in poor weather.

-- Maintain a safe stopping distance. Leave more distance than usual between your car and the car in front of you. Be able to stop within your sight distance.

-- Be alert for icy conditions, particularly on bridges and in shady spots.

-- Apply brakes early and gently. It takes longer to stop on ice and snow. Do not pump anti-lock brakes.

-- If the rear of your car slides or skids, steer slightly into the skid.

-- Turn headlights on low beam and use the wipers.

-- Keep windows defogged.

-- Accelerate slowly.

-- Do not use cruise control.

-- Be more alert. Watch for stalled cars and emergency and snow-removal vehicles.